What to measure
How to use the roofing calculator
Choose gable or hip and enter the length and width of the roof at the eaves, overhangs included, and the pitch. Set the waste (10–15%) and the bundles per square from the shingle wrapper. Enter how many 3 ft courses of ice barrier you’ll run at the eaves, and a price per square for the cost.
For the shingles alone, with starter strip bundles, use the shingle calculator. If you don’t know the pitch, the roof pitch calculator works it out from a rise and run.
The formula
- Roof area = length × width × √(1 + (pitch ÷ 12)²)
- Squares = roof area × (1 + waste) ÷ 100
- Underlayment rolls = squares × 100 ÷ 1,000, rounded up
- Ice barrier rolls = eave length × courses × 3 ft ÷ 200, rounded up
- Cost = squares × price per square
Worked example: 40 × 30 ft gable roof, 6/12 pitch
- 40 × 30 × 1.118 = 1,342 sq ft of roof
- 1,342 × 1.10 ÷ 100 = 14.76 squares → 45 bundles at 3 per square
- 1,476 sq ft ÷ 1,000 → 2 rolls of underlayment
- Eaves 2 × 40 = 80 ft × 3 ft = 240 sq ft → 2 rolls of ice barrier
- At $500 a square, $7,379.02
Roofing squares by footprint and pitch
| Footprint | 4/12 | 6/12 | 8/12 | Underlayment rolls |
|---|---|---|---|---|
| 30 × 24 ft | 8.35 | 8.85 | 9.52 | 1 |
| 40 × 26 ft | 12.06 | 12.79 | 13.75 | 2 |
| 40 × 30 ft | 13.91 | 14.76 | 15.86 | 2 |
| 50 × 30 ft | 17.39 | 18.45 | 19.83 | 2 |
| 60 × 32 ft | 22.26 | 23.61 | 25.38 | 3 |
Squares include 10% waste; footprints include the overhangs.
Common mistakes
- Measuring the floor plan instead of the eaves. Overhangs add a foot or two on every side, which is easily 10% more roof.
- Leaving out the pitch. The footprint is the flat area; the roof is bigger. At 8/12 it’s 20% bigger.
- Comparing quotes with different squares. Ask whether a quote’s squares include waste, and whether it is for materials or installed.
- Skipping ice barrier where code needs it. In areas with ice dams, the IRC (R905.1.2) requires an ice barrier from the lowest roof edge to at least 24 in inside the exterior wall line. With a deep overhang that can take two courses.
Frequently asked questions
What is a roofing square?
100 sq ft of roof surface. Roofers price and order by the square, so a 1,500 sq ft roof is 15 squares before waste.
How do I calculate roof squares?
Multiply the footprint at the eaves by the pitch factor, add waste, and divide by 100. A 40 × 30 ft roof at 6/12 is 1,200 × 1.118 = 1,342 sq ft; with 10% waste that's 14.76 squares.
How many squares is a 2,000 sq ft house?
It depends on the footprint and the pitch, not the living area. A 2,000 sq ft one-story footprint at 6/12 has about 2,236 sq ft of roof, or 22.4 squares before waste; a two-story house of the same living area has about half that.
How much underlayment do I need?
One roll of GAF FeltBuster covers 1,000 sq ft, so about one roll per 10 squares. The calculator counts rolls from the squares with waste, which also covers the laps between courses.
Where does ice and water shield go?
Along the eaves, in cold climates where ice dams form, and in valleys and around chimneys. GAF WeatherWatch comes in 3 ft wide rolls of 200 sq ft; set how many courses you'll run up from the eaves, or 0 where you don't need it.
How do I work out the cost of a new roof?
Get a price per square from your contractor or supplier and multiply by the squares with waste. The calculator does this when you enter a price, either installed or for materials only.
Does a hip roof need more material than a gable?
The same area for the same footprint and pitch, but more waste from the cut hips, more ridge cap and more eave for ice barrier. A gable roof has rakes that need drip edge and starter instead.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.